With the continuing decrease of technology node, demands for pattern size precision and overlay accuracy are increasing. However, during the lithographic process, due to the resolution limitation of optical imaging, the right-angled pattern corner in the photomask layout will be inevitably subjected to rounding distortion when exposed and imagined on a silicon wafer. If not being corrected properly, such corner distortion will cause various problems like size reduction of the pattern corner, which will bring adverse effects on the overlay accuracy and the pattern coverage rate. More seriously, such rounding distortion may also convert patterns which conform the design rules and designed to be safe in the layout into hotspots with insufficient process window. In order to avoid the pattern corner distortion, an Optical Proximity Correction (OPC) is generally adopted in the industry to perform photomask correction. The OPC correction method in the prior art mainly includes a rule-based OPC using serif correction and a model-based OPC.
The conventional OPC method comprises the steps of adding assist patterns near the target patterns, performing segmentation to the target patterns, moving the segments of the target patterns according to the difference between their simulation contour and the target patterns until the simulation contour is consistent to the target patterns. However, the size of the segments and the design of the assist patterns mainly depend on experience, which may easily cause hotspots due to the improper segmentation and assist patterns.
In order to improve the OPC correction precision, an inverse lithography technique (ILT) tool called PXOPC is developed. The PXOPC is a target-layer oriented tool which performs photomask optimization through curving the contour of the target patterns, forming photomask pattern with curvy contour from the target patterns, converting the curvy contour of the photomask pattern into a combination of straight lines by a differential method, and adjusting segments of the straight lines. Since the ILT-optimized photomask is generated through curving and re-linearizing, it can automatically optimize the assist patterns and perform proper segmentations, which helps to solve the hotspot problems. Although the PXOPC has the above advantages, the movement and simulation of the curvy contour of the photomask pattern demands enormous calculation capability. For example, the calculation time is four times greater than that of the conventional OPC method with the same number of CPUs, thus global correction cannot be performed.